1. Field of the Invention
The invention relates to a storage system for pallets, which comprises a pallet shelf with one or more carriages and a front transport device, the storage shelf being constructed from columns which are connected to one another by means of longitudinal and transverse girders, with the result that channels for pallet storage are obtained in each compartment at different supporting levels, and there being mounted on the transverse girders of the shelf in the longitudinal direction of each channel rails on which is movable through the channel in the longitudinal direction a carriage equipped with running rollers and with a drive for these, and which is equipped with devices for lifting a loaded pallet.
2. Description of Related Art
Various known storage systems come under this definition which corresponds to what is prior art. Before these prior art systems are discussed, however, some comments must be made to give a clear understanding of the storage conditions in systems with channel storage with regard to the system known as "serial shelving".
Serial shelves are constructed from columns which are interconnected substantially only by means of longitudinal girders. Between these there are storage compartments which are free of cross-connections, with the sole exception of the connections above the topmost stacking levels, since, of course, these connections are necessary for ensuring that the system as a whole has sufficient stability. However, in the compartments, the complete height of all stacking levels is freely accessible to a fork-lift truck equipped with an extendible mast, by means of which the pallets can always be set down on the desired level or lifted off from it. Although serial shelves are suitable both for the LIFO method and for the FIFO method, in the former case, of course, when there is an aisle on both sides of the compartment depth, the most important restriction is that they are suitable only when the number of pallets used to store the same article is large enough for an entire compartment to be provided for these; otherwise, the loss of filling capacity becomes too great. Storage and removal can be carried out only if each pallet location (the floor space above which pallets can be arranged at all levels) is filled up or cleared out completely in series. Finally, fork-lift trucks are not capable of extending their fork with regard to the mast sufficiently far to ensure that the latter reaches further than the first attainable pallet location, or at most, the second if a "reach truck" is used. If the number of pallets per article, on which the storage concept has to be based, is not so high, it is expedient to limit the channel depth. Two divisions can then be erected back-to-back with aisles on both sides, but this once again entails the restriction to the LIFO method, thus affecting the storage and removal sequence.
Series shelves also have other restrictions. The fork-lift truck always has to travel into and out of the complete free depth of each compartment with its loaded pallet. At the same time, the lateral clearance is in practice always limited to a few centimeters. The truck driver therefore has to move, very carefully in order to avoid collisions with the shelf, and this means that the lifting vehicles alway work slowly.
There is also the aspect of danger. Although a fork-lift truck is generally not allowed to travel with the load in the lifted-up position because of the reduced stability which then arises, this is nevertheless unavoidable with series shelves. In particular, the pallets are set down on short girder pieces which, in relation to the columns or in relation to the longitudinal girders (on both), are projecting into the compartment over a short distance in the transverse direction. This distance, that is to say the length of these short sections, is of course chosen so that a pallet set down on them is always supported on both sides. It is thereby impossible to lift a pallet inside the compartment. As a result, the pallet has to be lifted in the aisle to a short distance above the intended level, whilst the lifting vehicle in this condition has to enter the particular compartment to its full depth. It is clear that when work is carried out at higher levels, increasing accuracy is necessary when working with the lifting vehicle, and once again this has an adverse effect on the working speed.
Finally, with series shelves there is also the problem that there are many types of lifting trucks, of which the width at the top is too great to fit with sufficient clearance between the supporting girders. This problem can be avoided only arranging the second level so high that the lifting vehicle fits under the girders. Space is thereby wasted above the lowest level (where the pallets are put down on the floor). The alternative is to be restricted in the choice of the type of lifting truck, specifically to those which are so low that the second level can be arranged, at such height as determined by the dimensions of the loaded pallets, without loss of space.
Mainly in view of these considerations in respect of series shelves, there is a need for systems in which work can be carried out in channels, specifically at a working speed which is not restricted by maneuvering problems inside the compartments. The term "channel" or "tunnel" then refers to the entire space available at the same level over the full depth of a compartment between the rows of columns, and where appropriate the distance between these rows of columns can be chosen to be so great that there is room for two channels located next to one another on the same level.
A system of this type is known as defined in the first paragraph above relates, in which a carriage can travel completely underneath the pallets in the rails. For this purpose, the pallets stand on the rails and the rails are designed as horizontal U-sections, the wheels of the carriage moving within this U-shape. Mounted within the height of the carriage, i.e. also within the height of the horizontal U-section, is, in addition to a drive, a lifting mechanism for a table which is essentially exactly as large as the carriage itself and by means of which a pallet can be lifted until it comes free of the rail and can be shifted by means of the carriage.
Two alternative versions of a front transport device are proposed. One is a hoist within the space of a pallet location in the shelf, which interacts with a distributor carriage at each level outside the shelf, with the result that a pallet can be brought to this level in front of the respective channel. The actual channel carriage ensures that the gap relative to the rails in the shelf is bridged. The other alternative version is a mast which is equipped with a lifting table and which moves in a rail relative to the floor. This lifting table too has to bridge a gap relative to the rails in each channel.
The carriage travelling in the channels are independent; a computer-controlled locating and storing system is provided. The hoist and distributor carriage in the first version and the travelling column with the lifting table in the other version are also controlled in this way.
A consideration problem of these systems, which are also known as satellite systems, is that the height of the rails or of the carriage within the system as a whole constitutes a loss factor which is repeated for each level. Furthermore, to ensure the desired independence, each carriage contains its own battery. This too constitutes a factor which increases the costs considerably.
This prior art storage system is very expensive for various reasons. The shelf construction is a special, heavy construction, the automatic control system which has to work with a high degree of accuracy also entails high basic costs. In practice, then, the system can be used ecomonically for very large stores only, mainly in manufacturing plants. It is pointed out, for the sake of completeness, that the system allows the use of both the FIFO method and the LIFO method for storage and removal.
Through-type shelves and running-pallet shelves are also known. The former are equipped with roller tables both in a non-driven version and in a driven version. These allow only the FIFO method, and therefore, space is required for an aisle both at the front and at the rear. Roller tables are somewhat expensive, and this is even more true of driven roller tables. If they are not driven, there are considerable restrictions as regard use (gradients, load stability, pallet design, channel length, dynamic pressure, etc.) which, moreover, partly apply also to the driven versions.
The running-pallet shelves have rails extending in the longitudinal direction of the channels, but a movable underframe is necessary for each pallet. Although there is the choice of setting a standard pallet down on such an underframe or stacking the load directly on a special pallet equipped with wheels, it is clear, nevertheless, that both are cumbersome and at all events result in a very expensive system.
Through-type shelves and running-pallet shelves certainly both have the advantage that the work can be carried out more quickly, because, after the pallet has been set down, it is not necessary to wait until the pallet has reached its location in the stores. These systems can be advantageous particularly for fields where a large number of pallets have to be handled daily.